1
Pacemakers for A nesthesiologists M ade Incredibly Simple
by Arthur Wallace, M.D., Ph.D.
April 2008
What do pacemakers do?
a. Pace the heart
b. Sense the heart
How do you pace the heart? You send an electrical current to the heart. The current is
small and it is measured in milliamps (ma).
How do you sense the heart? The pacemaker measures the voltage produced by the heart
when it contracts. The voltage is small, and is it measured in millivolts (mv).
What is an electric current? An electric current is the flow of electrons. The pacemaker is
a drug delivery device. The drug delivered is electrons. The current is the dose of
electrons per second.
A milliamp is a small current. When you start your car you use tens to hundreds of amps.
The current the pacemaker is delivering is one thousandth as big as the current to start
your car.
What is a voltage? Remember when you were in high school and there was a smart kid
!"#$%&'$&(#)*+$+#,*-$*#'",*-.$/0&1"0(%$!#)2+$%&3$'",*-%$2,405$670$"&%$&$2#'$#8$
9#'0*',&25:$;201'rical voltage is the same thing. Voltage is the potential to do something.
<'$+#0%*='$"&>0$'#$+#$&*3'",*-?$@)'$,'$1#)2+?$,8$,'$802'$2,40$,'5$A#)$1&*$"&>0$&$@&''0(3$!,'"$
1.5 volts, or 12 volts, or some other voltage. It can just sit there waiting to do something.
Current implies the delivery of electrons (the dose per second). Voltage is simply the
potential of those electrons if you bothered to deliver them. The electron in the diagram
@02#!$"&%$&$9#'0*',&2$(02&',>0$'#$-(#)*+5$$B)'$,'$+#0%*='$"&>0$'#$+#$&nything. The symbol
for the ground is the little triangle with the line hooked to it.
What is a ground? It is the earth. If you go outside your house to the power meter and
you look at the ground near the power meter, there will be a bare copper wire that is
screwed onto a copper post shoved in the ground. Your house is attached electrically to
the ground with this wire and metal post. This is essentially the sewer for spare electrons
from your house. The ground is the electron dump.
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Electron
Potential
Ground
So what is the big deal about this voltage C current stuff? D022?$(0E0E@0($F"E=%$G&!.$
A#)$(0E0E@0($8(#E$9"3%,1%$12&%%$'"&'$HI<J?$@)'$@08#(0$!0$-#$#>0($F"E=%$G&!$20'$E0$
E&40$'",%$120&(5$K#*='$0>0($>,#2&'0$&$GLD$#8$9"3%,1%5$D"0*$3#)$+#?$'"0$90*&2',es are
severe. Physics theories - OK. Physics LAWs - +#*='$+#$,'5
Ohms Law: V=IR
V: Voltage
I: Current
R: Resistance
D"&'$'",%$2&!$%&3%$,%$'"&'$'"0$1)((0*'$+090*+%$#*$'"0$>#2'&-0$&*+$'"0$(0%,%'&*105$G0'=%$+#$
a little experiment. Here are two plugs. I attach wires to a patient, like ECG leads. I hook
the wires to the ground of two separate electrical plugs. Can I hurt the patient with this set
up?
Voltage
Ground
Left Plug GND = 0.1 V
Right Plug GND = 0.2 V
G0'=%$E&40$&*$&%%)E9',#*5$/"0$-(#)*+$#*$'"0$208'$,%$*#'$(0&223$M5$<'$is at, say, 0.1 volts.
L*+$'"0$-(#)*+$#*$'"0$(,-"'$,%$%2,-"'23$+,880(0*'?$20'=%$%&3$M5N$>#2'%5$/"&'=%$9(0''3$12#%0$'#$
O0(#5$P#)*+%$8,*0?$(,-"'.$/"0$(0%,%'&*10$#8$#)($!,(0%$!,22$@0$2#!5$G0'=%$E&40$'"0$E&'"$
easy and say we purchase really, really good wire with a resistance of 0 ohms.
G0'=%$'&40$F"E=%$G&!Q$H$I$<J5$$D0$(0&((&*-0$,'$'#$E&40$<$I$HRJ
I
I
V1 V 2
R
0 .2 0 .1
0
3
Remember that something divided by zero is infinity. And infinity is a very, very big
number. When infinity is the current sent through your body, strange things happen, like
smoke, charring, burns, and ultimately transfer to the eternal care unit. This is called a
ground loop. The electrical equipment in your operating room is specially designed to
prevent ground loops. For example, none of the ECG wires that you hook to patients is
really connected to the wall. They are electrically isolated from the power in the wall.
The wall power is electrically isolated from the electric company. The electricity in the
OR is Special E D E lectricity to prevent this type of injury. But when you put a wire on a
9&',0*'?$#($&$!,(0$,*'#$&$9&',0*'=%$"0&('$S!",1"$,%$!"&'$&$9&10E&40($(0&223$,%T$E&40$%)(0$
3#)$+#*='$+#$%#E0'",*-$'#$#>0(1#E0$'"0$9(#'01',#*%$@),2'$,*'#$'"0$0201'(,1&2$,%#lation.
Special ED Electricity and the OR
Electrical Isolation is designed to make it more difficult to burn, electrocute, or
incinerate stuff (patients) in the OR. Do not attempt to bypass the special systems or
electrical isolation monitors or checking done by bioengineering. The ECG wires are
not physically connected to the monitor. They are optically isolated from the
monitor, which means they send a light signal, h (photons), to the monitor. The
0201'(#*%$8(#E$'"0$9#!0($1#E9&*3$1&*='$-0'$,*'#$'"0$9&',0*'$'"(#)-"$'"0$E#*,'#(5
ECG
H
Monitor
ECG:
1 mv
Voltage
Time
Ok, so what is that? It is a plot of Voltage on the Vertical axis (vertical is up and down)
and time on the Horizontal axis (left and right).
What is that little box on the far left? How big is that? First, the units. The little box is
measured in millivolts. It is 1 millivolt (1 mv) high. So the Q RS complexes are
approximately 1 millivolt in amplitude (height).
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If I tell you that the heart rate is 60 beats a minute, how many seconds is it between
beats?
UE?$)E?$!022$VM$+,>,+0+$@3$VM$!#)2+$@0$WX$A0%?$'"&'=%$(,-"'5$<'$,%$#*0$%01#*+$@0'!00*$
heart beats if the heart rate is 60. Ok, so the R-R interval, the time between QRS
complexes, is 1 second if the heart rate is 60. How many milliseconds is that? Correct,
1000 milliseconds is equal to 1 second. Now, how long is the PR interval? Less than 200
milliseconds if the patient does not have first degree heart block. How long is the Q T
interval? Right, it is approximately 400 milliseconds.
1.0 mv (one millivolt)
Voltage (mv)
R-R Interval 1000 msec (1000 millisecond)
Time
QT Interval 400 msec
PR Interval 200 msec
How does the pacemaker detect an R wave? It detects the baseline. Then it sets a level, a
voltage level above which it says there is an R wave. If the voltage of the ECG exceeds
that level, it is an R wave. Normally this voltage level would be called a threshold. The
9(#@20E$,%$'"0$!#(+$6'"(0%"#2+:$,%$(0%0(>0+$,*$9&10E&40($'0(E,*#2#-3$8#($'"0$E,*,E)E$
current to pace the atrium or ventricle. So, the term THRESHOLD is a current. It refers
to the minimum current to pace the >0*'(,1205$A#)$1&*='$)%0$'"0$!#(+$/7J;P7FGK$'#$
describe this voltage level. So in pacemaker lingo, the word SENSITIVITY is used.
SENSITIVITY is the voltage level that must be exceeded to detect an R wave or a P
wave.
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Voltage Level Above Which It Decides there is an R wave ( SE NSI T I V I T Y (mv) )
R
R
R
R
R
R
R
R
R
RRRRR R
R
Baseline Voltage
Now here is a little problem. You have a patient with a pacemaker. The pacemaker is not
detecting the R waves. You decide to make the pacemaker MORE sensitive, so that it
will detect the R waves. What do you do? You lower the SENSITIVITY in millivolts, to
make the pacemaker MORE sensitive. Should I say that again? It makes sense if you
think about it in voltages. To make the pacemaker more sensitive, L OW E R the voltage
of the SE NSI T I VI TY.
Before: Not Sensing R Waves, Make it MORE sensitive to R waves.
After: Sensing R Waves, because SENSITIVITY in millivolts was lowered.
R
R
R
R
R
R
R
R
R
RRRRR R
How do I detect a P wave? Same concept. Establish a baseline. Determine a level called
'"0$L'(,&2$P;YP</<H</A5$<8$'"0$>#2'&-0$0Z100+%$'"0$20>02?$,'$,%$&$[$!&>0.$Y#?$'"&'$!#*='$
work, because there are also T waves. What do I do about the T waves? Find an R wave,
wait a refractory period, and the next thing along is a P wave.
R
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Refractory Period
p
p
p
R
R
R
p
Ugly Smudge?
That works. Now, what is that smudge on the ECG above? Am I a spaz? Or just a lousy
artist? What is that smudge?
The ugly smudge is noise: Electrical Noise. In the OR, the noise is usually from the
electrocautery, also known as the bovie. What does the pacemaker do when it sees
electrical noise such as the bovie?
R
R
R
R
R
R
R
R
R
RRRRR R
R
Electrical Noise Caused by Electrocautery (Bovie)
The pacemaker is inhibited by electrocautery (the bovie). The pacemaker measures
voltages. It thinks that smudge is electrical activity from the heart. When it detects
electrical activity from the heart, it is programmed to NOT PACE. The word INHIBITED
in pacemaker land has nothing to do with the sexual proclivities or abilities of the
pacemaker. The word INHIBITED in pacemaker lingo means, DOES NOT PACE. If the
pacemaker is INHIBITED it just sits there doing nothing. The word inhibited does NOT
imply that the pacemaker can inhibit the heart. The pacemaker paces. The pacemaker
does not inhibit. The pacemaker can only make the heart go FASTER. The pacemaker
can not make the heart go slower. The pacemaker CAN be inhibited by electrical activity
of the heart. The word INHIBITED refers to the pacemaker NOT the heart.
G0'=%$1#*%,+0($&$1&%05$A#)$"&>0$&$9&',0*'$!,'" 3rd degree heart block (the conduction
%3%'0E$,*$'"0$9&',0*'=%$"0&('$+#0%*='$!#(4T5$/"0$9&',0*'$"&%$&$KKK$9&10E&40($,*$92&105$
The first D in DDD is the area PACED. The D stands for DUAL, so this pacemaker,
paces both the atrium and the ventricle. The second letter stands for Area SENSED, so
this pacemaker senses both the atrium and the ventricle. The third letter stands for what it
does, and the D stands for Dual inhibited. This pacemaker can be INHIBITED by
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electrical activity from an atrial contraction and it will not pace the atrium. Or, it can be
INHIBITED by the electrical activity of a ventricular contraction, and not pace the
ventricle, or both. It has a lot of inhibitions. So, you take the patient in the operating
room and the surgeons starts to use electrocautery. They use the electrocautery
continuously. What is the heart rate and the blood pressure of the patient?
Well, the pacemaker detects the electrical noise from the electrocautery. It thinks there is
an R wave. The pacemaker is inhibited, s#$,'$+#0%*='$9&105$<8$'"0$9&10E&40($+#0%*='$9&10?$
'"0$"0&('$+#0%*='$@0&'5$/"&'$!#)2+$E&40$'"0$"0&('$(&'0?$O0(#5$D",1"$!#)2+$E&40$'"0$
blood pressure, zero. Many experts agree that chronic beating of the heart is good, and a
blood pressure of zero is bad, but these finding have not been confirmed in prospective
randomized trials.
So, if a heart rate of zero is BAD, what do you do?
Right, you set the pacemaker to DOO. The first letter is the area paced. So, Dual pacing
would pace the atrium and the ventricle. The second letter is the area sensed, so the O
E0&*%$,'$+#0%*='$%0*%0$#($,'$"&%$YF$P;YP;5$/"0$'",(+$20''0($,%$'"0$<Y7<B</<FYP5$/",%$
pacemaker has NO INHIBITIONS. DOO stands for DUAL pacing with NO SENSE and
NO INHIBITIONS. It sounds a bit like Paris Hilton.
DOO and VOO
NO SENSE
NO INHIBITIONS
The Paris Hilton of Pacing
Modes.
Now the pacemaker completely ignores the electrocautery. The pacemaker paces and the
heart beats. The blood pressure returns. The case finishes. You send the patient to the
intensive care unit. About an hour later, the patient is awake, He warms up a bit. And you
hear this on the monitor.
Beep, Beep, Beep, Beep, Beep, BeBeep,,, Beep, Beep, Beep, Beep, Beep, BeBeep,,, The
("3'"E$,%*='$\),'0$(,-"'5$/"0(0$,%$&*$0Z'(&$@009$,*$'"0(05
You look up at the monitor and this is what you see.
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Extra beat
Extra beat
R on T
VF
As you begin doing CPR and calling for the defibrillator, you think, what did I do wrong?
Well, You heard the extra beat. The rhythm was not beep, beep, beep, beep, beep, It had
an extra beat in there. That extra beat occurred earlier than expected and the pacemaker
then paced on the T wave of the extra beat causing an R on T. The R on T caused
ventricular tachycardia (VT) and then ventricular fibrillation (VF). Ooops!!!!!!!!
The entire point of a pacemaker sensing is to avoid R-on-T. When you select an
asynchronous pacing mode such as VOO or DOO, you are the sensor. The pacemaker is
NOT sensing the electrical activity of the heart. You are responsible for sensing. If you
detect extra beats, turn the pacing mode to one that senses such as DDD or VVI. DO
NOT LEAVE THE PACEMAKER IN DOO or VOO when you are not in an OR with
electrocautery that is actually causing a problem with the pacemaker.
Never leave a patient in DOO or VOO.
Now, here is a trivia question. Have you ever seen a James Bond Movie? What is James
B#*+=%$1#+0$*)E@0(.$MM]5$D"&'$+#0%$'"0$+#)@20$MM$,*$MM]$%'&*+$8#(.$
LICENSED TO KILL. When you leave a patient in DOO or VOO, it is LICENCE TO
KILL. DO NOT LEAVE A PATIENT IN DOO OR VOO mode.
When you leave a patient in the ICU, check the pacing mode on the pacemaker and
record it on your anesthesia record. If you forget to write it down there is a problem. An
hour later someone may change the pacing mode to VOO or DOO. Then when the patient
"&%$H/$&*+$H^?$3#)$!,22$@0$@2&E0+5$<8$3#)$!(,'0$'"0$E#+0$+#!*?$3#)$!#*='$8#(-0'?$'"0*$
3#)$!#*='$@0$@2&E0+$8#($20&>,*-$,'$,*$HFF$#($KFF5$
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D O N O T L E A V E A PA T I E N T I N D O O or V O O .
The OO modes are only for use in the operating room when there is electrocautery that
interferes with the pacemaker.
What does the Magnet do?
Medtronic Dumb
Magnet
Medtronic Smart Magnet
Before we get into Magnets, there are a few baseline questions.
Question 1: Are Magnets Magic?
It ,%$,E9#('&*'$'#$&%4$'",%$@01&)%0$,'=%$*#'$)*1#EE#*$in pacemaker-2&*+$'#$"0&(?$6F"?$
+#*='$!#((3$&@#)'$'"0$9&10E&40(5$_)%'$9)'$&$E&-*0'$#*$,'5:$#($6<8$'"0(0$,%$&$9(#@20E?$<=22$
9)'$&$E&-*0'$#*$,'5:$D"0*$3#)$"0&($'",%$E&-,1&2$'",*4,*-?$'"0$*0Z'$'",*-$3#)$%"#)2d ask
,%?$,8$E&-*0'%$E&-,1&223$%#2>0$&22$9&10E&40($9(#@20E%?$!"3$+#*='$'"03$`)%'$9)'$&$E&-*0'$
permanently on the pacemaker? Or better yet, sell a magnet with each pacemaker and
have the patient wear the magnet in a special shirt pocket magically precluding all
pacemaker problems?
The magnet is NOT MAGICAL. The magnet simply throws a programmable switch in
the pacemaker. It changes it from one mode to another. The magnet mode depends on the
brand of pacemaker, the type of device, whether magnet mode is turned on, and it may
even depend on the level of battery charge in the device. In some implantable devices the
magnet mode may shut off the device. In others it may make the device listen for
information from a radio frequency controller. It may change modes to another type of
function. The magnet throws a programmable switch. It may or may not be what you
want.
Question 2: !"##$%&'($"')*+'*',$-#".&/'0)&/&'.)&'#*.1&2.3%'415&'+&#&2+&+'$2'.)&'
function of the computer. If the computer malfunctions, the patient dies. Would you go up
to the computer keyboard and randomly punch keys to see what would happen?
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Of course not!!!! No one in their right mind would randomly punch buttons on a
1#E9)'0($'"&'$&$9&',0*'=%$2,80$+090*+0+$#*5$
Question 3: Should you randomly throw a switch in an implantable device without
knowing what the switch does?
The magnet throws a switch. If the implantable device is a pacemaker it frequently will
put the device in DOO at 99. If the implantable device is an implantable cardiac
defibrillator (ICD), it may shut off the defibrillation functions. If the device is a deep
brain stimulator, it will do something else. If it is a vagal nerve stimulator, something else
will happen. If it is a dorsal column stimulator, it will be yet something else. If it is a
bladder stimulator, the patient may pee on your shoes. The point is simply that the
magnet throws a switch and you should know what the device is and what the magnet
does before you randomly throw it.
There was a study done at a large Midwe%'0(*$"#%9,'&2$%3%'0E$!"0(0$'"03$)%0+$'"0$6"#90$
8#($'"0$@0%'$&*+$,8$'"&'$8&,2%$9)'$&$E&-*0'$#*$,':$&99(#&1"$'#$9&10E&40(%5$/"0(0$!&%$&$ab$
operative mortality associated with pacemaker function. They started interrogating all
pacemakers prior to surgery and found that 10% were not functioning properly. They
then began interrogating all pacemakers prior to surgery and completely eliminated
perioperative pacemaker associated mortality.
I went to a code for an elderly gentleman admitted to the medicine service with
congestive heart failure. I looked at this very short of breath man and there was a blob
under the skin inferior to his left clavicle. I pointed to the blob under the skin and asked
'"0$E0+,1&2$(0%,+0*'?$6D"&'=%$'"&'.:$/"0$(0%9#*%0$!&%$6D0$'",*4$,'$,%$&$9&10E&40(5:$<$
(0%9#*+0+?$6/"0$9&',0*'$"&%$@00*$,*$'"0$"#%9,'&2$8#($'!#$+&3%$!,'"$1#*-0%',>0$"0&('$
8&,2)(0$&*+$3#)$+#*='$4*#!$!"&'$'"&'$+0>,10$,%.$K,+$3#)$-0'$&$1"0%'$Z-ray on this
&+E,%%,#*.:$6F"$30%?$+08,*,'023:$6L*+$!"&'$+,+$,'$%"#!.:$6<*8,2'(&'0%?$1urly B lines,
1&(+,#E0-&235:$6c#)2+$3#)$%00$'"0$@2#@$)*+0($'"0$%4,*.:$6Ugh.:$6Was there a lead to
'"0$&'(,)E$&*+$&$%01#*+$20&+$'#$'"0$>0*'(,120.:$6U-".:$6D0(0$'"0$20&+%$@(#40*.:$6U-".:$
6K,+$3#)$"&>0$1&(+,#2#-3$,*'0((#-&'0$'"0$9&10E&40(.:$6U-".:$6<%$,'$8)*1',#*&2.:$6U-".:
The blob under the skin was a pacemaker, it was malfunctioning, and unfortunately the
patient arrested and died prior to us being able to correct the pacemaker malfunction.
Pacemakers are just electronic devices situated in a hot, salt water filled environment,
with motion. This environment is NOT compatible with electronics. Despite the hostile
environment they work pretty well but NOT perfectly. If you have a question about a
patient, interrogate the pacemaker and figure out if it is working. It may not be
functioning properly.
The next thing that drives me nuts is physicians who take a patient in the operating room
without knowing the brand, model, function or magnet mode of the pacemaker, when it
was last checked, who controls it, whether the hospital has a programmer for this make
and model of pacemaker, whether there someone who can run the programmer, and what
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'"0$,*+,1&',#*%$&(0$8#($'"0$9&10E&40(5$/"03$!,22$%&3$%#E0'",*-$2,40$6F"?$,'$!&%$&*$
emergency, and if there was a problem, we w#)2+$9)'$&$E&-*0'$#*$,'5:$/",%$,%$E&-,1&2$
thinking. The magnet simply changes mode. If the new mode is better than the mode you
&(0$,*?$-(0&'?$,8$*#'X
It takes about five minutes to find out everything there is to know about a pacemaker.
Patients carry a 1&(+$,*$'"0,($!&220'$'"&'$-,>0%$&$2#'$#8$,*8#(E&',#*5$$<8$3#)$1&*='$8,*+$'"0$
card you can get an X-ray of the device and see the make on the circuit board. Moreover,
you can see the leads and tell what the device is. The easiest way to find out what the
device is, is to call Medtronic Bradyarrhythmia Products at 800-505-4636. The phone
number is on the web at http://www.medtronic.com under the physician section for
electrophysiology. They are open 24/7/365. If you call and identify the patient, they will
'022$3#)$,8$'"0$+0>,10$,%$&$d0+'(#*,1$+0>,105$<8$,'$,%*='?$'"03$E&3$0>0*$-,>0$3#)$'"0$9"#*0$
number for the other manufacturers such as Guidant, etc. These companies want to help
you. They want to solve the problem quickly. They make a lot of money on these devices
and they want you to be pleased with their service. If there is a question with a pacemaker
or defibrillator they will send a rep. The rep is usually one of the most intelligent,
beautiful reps you will meet and even better, they may bring donuts. If in doubt just call
them and they will help you, and they may bring donuts.
What you need to know:
a.
b.
c.
d.
e.
f.
What is the device?
What brand and model?
Who controls it?
Does your hospital have a programmer for this make and model?
What is the magnet mode?
Why does the patient have a pacemaker? This is important because if the
patient has it for third degree block, you should treat the device as if the
9&',0*'=%$2,80$+090*+%$#*$'"0$+0>,10?$@01&)%0$,'$+#0%5
g. What rhythm does the patient have when the pacemaker is shut off? If the
rhythm is asystole, treat the device with a certain respect and caution. If you
plan to replace the device, you may want to place a temporary wire to pace
while the implantable pacemaker is not functional.
One approach to finding out about pacemakers is to go to the electrophysiology clinic
(EP) that controls the pacemaker and ask about it. You should be careful though. When
3#)$%&3$%#E0'",*-$2,40$6D"&'$+#0%$'"0$E&-*0'$+#.:$/"03$!,22$%&3?$6F"?$,'$&2!ays goes
'#$KFF$&'$ee5:$d3$(0%9#*%0$,%$)%)&223?$6<%$'",%$'()0.:$/"03$!,22$'"0*$2##4$&'$3#)$
\),OO,1&223$&*+$%&3$6<$+#*='$4*#!5:$/"0$(0&%#*$8#($'",%$1#*8)%,#*$,%$80!$'",*-%$&(0$
always true and the DOO at 99 for magnet mode is NOT ALWAYS true. Let me give
you an example.
D"0*$&$9&10E&40(=%$@&''0(3$()*%$+#!*$,'$-#0%$,*'#$%#E0'",*-$1&220+$;YK$F^$G<^;$
MODE. This is a very poorly worded mode. It really should be called END OF
BATTERY LIFE MODE, but END OF LIFE MODE is what they call it. When a
12
9&10E&40(=%$@&''0(y runs down, it tries to save power, so in some devices the MAGNET
mode for END OF LIFE MODE is DOO at 50. What happens to a patient that has an
,*'(,*%,1$"0&('$(&'0$#8?$%&3?$*#(E&2$%,*)%$("3'"E$&'$]M?$!"0*$3#)$9&10$KFF$&'$fM.$/"&'=%$
right, you get VT then VF then END OF LIFE MODE. Be careful with the magnet, it
throws a switch, It is NOT MAGICALLY, SPECIALLY, WONDERFUL - it simply
throws a switch.
Intrinsic Heart Rate
80
50
50
80
Pacemaker Rate
50
80
80
50
Pacemaker Mode
DDD
DDD
DOO
DOO
Result
80
80
80
VF (This is BAD)
/",%$,%$&$(0&223$,E9#('&*'$1"&('5$[&10E&40(%$&11020(&'0$'"0$"0&('5$/"03$+#*='$0>0($%2#!$,'$
down. You should think of the pacemaker as poking the heart. You can only speed it up
@3$9#4,*-5$A#)$+#*='$%2#!$,'$+#!*$@3$9#4,*-5$<8$'"0$9&10Eaker is in a mode that inhibits
%)1"$&%$KKK$#($HH<?$'"0$9&10E&40($1&*$%0*%0$'"0$"0&('=%$,*'(,*%,1$("3'"E?$&*+$,'$!,22$
avoid pacing on a T wave. If the pacemaker is in DOO or VOO mode, it does not
SENSE, it does not INHIBIT. It just paces. If the intrinsic heart rate is lower than the
paced rate, the pace maker will speed up the heart, and every thing will be OK. If the
pacemaker is slower than the intrinsic heart rate, the pacemaker will pace on a T wave
and you will get VT and/or VF and this is BAD. DO NOT PACE WITH DOO or VOO
unless you have electrocautery that interferes with the pacemaker. In DOO or VOO
mode, YOU are the sensor.
Which is a safe pacing mode: DDD or DVI?
G0'=%$'",*4$&@#)'$'",%5$KKK$,%$K)&2$9&1,*-?$+)&2$%0*%,*-?$+)&2$,*",@,',#*%5$KH<$,% dual
pacing, only senses the ventricle, is only inhibited by the ventricle. DVI ignores the atrial
1#*'(&1',#*.$7EEEE5$<$+#*='$4*#!5
13
If the R-R interval is 200
msec, the HR = 300 and
the blood pressure = 0.
p
DDD tracks atrial rates.
There is a MAXIMUM
RATE setting to prevent
1:1 conduction of
supraventricular
tachycardias.
Atrial
Flutter
1.
2.
3.
4.
Detect p Wave
Wait 200 msec (PR interval)
If no QRS, pace
Repeat
If a pacemaker is in DDD mode, it tries to follow the atrial rhythm. If the atrial rate
accelerates, the pacemaker will track it. If the atrial rate becomes too high, there is a
MAXIMAL RATE to prevent 1:1 conduction of supraventricular tachycardias which
would have rates that would preclude ventricular filling. 1:1 conduction of atrial flutter at
a rate of 300 would give a heart rate of 300 and a blood pressure of zero. DVI mode is
useful in situations where there are frequent supraventricular tachycardias or in situations
where the patient should not track the atrial rhythm.
C ASE: You are called to the ICU to see a patient with a pacemaker. The nurse is very
concerned because the pacemaker appears to not be functioning. The nurse has set the
pacemaker to DDD at 50 but you see the following on the monitor.
HR = 110
The pacemaker is set at DDD at a rate of 50. The monitor says the heart rate is 110. It
looks like 110. The pulse oximeter reads a rate of 110. The arterial line gives a rate of
110. You put your finger on the pulse and it reads 110. You look at the monitor and, sure
enough, there is a pacing spike on every QRS and the pacemaker is pacing at a rate of
110. Is the pacemaker working?
14
Yes. The pacemaker is working perfectly. The pacemaker is detecting the P waves caused
by sinus tachycardia. The pacemaker then wa,'%$'"0$%0'$[J$,*'0(>&2?$20'=%$%&3$NMM$E%015$<8$
there is first degree AV block and no QRS occurs spontaneously within the 200 msec,
then the pacemaker paces. It then waits for the next P wave, waits 200 msec, if no QRS, it
paces. It is following sinus tachycardia perfectly. If you want the heart rate to slow down,
3#)$+#*='$%0'$'"0$9&10E&40($%2#!0(?$3#)$-,>0$&$+()-$2,40$&$@0'&$@2#140(?$#($9&,*$
medication like fentanyl, or something that will slow the sinus tachycardia. The
pacemaker is doing exactly what it is programmed to do (DDD). It is following the atrial
contraction.
Which pacing mode is most efficient? Before you answer, let me define efficiency as the
highest cardiac output. One more caveat, the conduction system is intact, and the sinus
heart rate is too slow.
5
4
Pacemaker
to Right
Ventricle
7
8
3
2
6
6
9
7
10
1
8
10
2
8
9
3
7
4
5
6
7
8
The answer is AAI. Lowly atrial pacing, atrial sensing, inhibition by pacing. AAI has no
chance of pacing the ventricle on a ventricular T wave, reducing risk. But the most
important point is that AAI mode relies on the normal ventricular conduction system, so
the conduction is synchronous across the ventricular wall. In the above diagram the right
ventricular free wall is paced. The contraction passes slowly through the right ventricular
free wall to the septum and finally to the left ventricular free wall. The contraction is not
synchronous. The right ventricular free wall contracts prior to the septum and long before
the left ventricular lateral wall. The ventricle is less efficient by 10-15%. The reason that
15
paced QRS complexes are wider than normally conducted QRS complexes is because the
contraction is less synchronous. The reason that bi-ventricular pacing leads sometimes
help patients with congestive heart failure pacing with two leads, one in the right
ventricle and one passed through the coronary sinus towards the lateral wall, is that the
contraction is more synchronous and more concentric and more efficient. In a patient
with good ventricular function, 10-Wfb$+#0%*='$E&''0(5$B)'$,*$9&',0*'%$!,'"$2#! cardiac
output states, 10% is a lot. When your cardiac index is lousy, 10% may make the
difference between needing inotropes and not, or needing a balloon pump and not. 10%
matters when you are desperate to improve cardiac output.
C L I N I C A L C ASE: You are called to the transitional care unit for a CODE BLUE. You
find a patient in the bed in asystole. They have a dressing on their chest from a
sternotomy wound and there are four little white wires coming out of the skin just lateral
to the inferior edge of the sternotomy scar. You begin CPR, ventilate, oxygenate, give
epinephrine & atropine, and intubate. What next?
Sternal
Wound
Dressing
White Wires
D"&'$'#$+#.$/"0$!",'0$!,(0%$&(0$9&1,*-$!,(0%5$/"0$#*0%$#*$'"0$9&',0*'=%$(,-"'$-#$'#$'"0$
right atrium. The ones on '"0$9&',0*'=%$208'$-#$'#$'"0$>0*'(,1205$7##4$'"0$9&1,*-$!,(0%$'#$
'"0$0Z'0(*&2$9&10($&*+$')(*$,'$#*5$6D"&'$,8$<$+#*='$(0E0E@0($!",1"$,%$!",1".:?$3#)$E,-"'$
ask. Hook up the wires and turn it on. Prior to doing this the patient is DEAD in
ASYSTOLE. If you do*='$"##4$)9$'"0$!,(0%$'"0$9&',0*'$E&3$(0E&,*$K;LK5$7##4$)9$'"0$
!,(0%5$6<$+#*='$"&>0$'"0$0Z'0*%,#*$1#(+%gggg:?$3#)$E,-"'$%&35$P#$!"&'.$_)%'$'&40$'"0$E0'&2$
lead at the end of the little white wire and put it in the little hole at the top of the external
pacemaker and turn it on. The extension wires are NOT necessary to make it work.
16
Lock
Heart Rate
Atrial Output Current
Ventricular Output Current
Change Modes
On Switch
The extension wires are
NOT necessary. You can
just put the pacing wire
ends into the little holes.
17
Hole to stick pacing wire
in. Extension wire NOT
necessary
Change Heart Rate
Press & Hold to Change
Value Button
Change Output Current
Change Sensitivity
Voltage
Change Mode
On Buttons - Make
sure you also push the
Press & Hold to
Change Value Button
at the same time, i.e.
you need to push two
buttons at once to turn
it on.
Battery Compartment
Takes 9Volt Battery
Medtronic 5346 DDD Temporary Pulse Generator is an older model. It is not the easiest
device to use. There are a few points which will make it much easier to use. Hold the
device in your left hand. Put your thumb on the magic secret button marked Press &
Hold to Change Value. There is actually a diagram of your fingers and thumb on the
front to remind you. PUSH AND HOLD the Press & Hold to C hange V alue Button
any time you want to change a value. Then push the button for the thing you want to
1"&*-05$/"0*$)%0$'"0$)9$&*+$+#!*$&((#!%$'#$E&40$,'$1"&*-05$<8$3#)$+#*='$push and hold
the Press & Hold to Change Value Button, nothing will work. It is like a clutch in a car.
To change gears push in the clutch (magic secret green button). You should play with
this device prior to using it in an emergency to get the concept of how to change values.
The screen only displays the values that matter in the mode you are in when you are in a
mode. For example in AOO mode only the heart rate and atrial output current will
display. In DDD mode everything displays. It is a good device, it just takes a couple of
minutes to figure out.
18
Bipolar Versus Unipolar
What is an electric circuit? What is bipolar? What is unipolar?
When you think about medical devices a big deal is made of unipolar versus bipolar. Is
there some fundamental difference? No, they are not really different. They use the same
electrons. They use the same physics. Bipolar devices put the ground near the source.
Unipolar electronic put the ground somewhere else.
Electrons are electrons. There is only one type. The basic concept of electronic circuits
can be described as ELECTRONS are SHY. If an electron cannot find its way home from
'"0$9&('3?$,'$!#*='$-#5$;201'(#*%$!&*'$&$(0')(*$9&'", they want a circuit. They only buy
round trip tickets. You try to sell them a one way ticket, they will not go. So, if you want
an electron to do something you need a return path, you need a circuit!
Current Source (Unipolar
Electrocautery)
Current Source (Bipolar
Electrocautery)
Brain
e
e
Grounding
Pad on Butt
Why do the neurosurgeons use a bipolar bovie and the rest of surgeons use unipolar
bovie? The neurosurgeons want to control the current path. They do not want to use the
brain stem as a ground. The bipolar electrocautery gives the return path to the electrons
%#$'"&'$'"03$+#*=' use the brain stem, or the spinal cord for a return path. Unipolar bovie
might cause injury or a seizure.
Why do the neurosurgeons drip saline on the bovie? They are trying to dissipate the
current even more. They just want to coagulate between the tips of the forceps.
62'*'"21#$4*/'7$81&9'0)('+$&%23.'.)&':/$"2+'#*+'$2'.)&'7"..',*"%&'*'7"/2; The area of
the bovie point is very small. The area of the grounding pad is large. The current density
on the bovie tip is high, causing heating and coagulation. On the grounding pad the
current density is low, preventing burning. The nurses check the grounding pad to make
sure it lies flat and has good contact throughout to prevent high current density and burns.
19
<)('+$&%23.'.)&'7$81&'%.$#'.)&')&*/.;' The current in the electrocautery is many, many
amps, like 10 amps. The voltage in the electrocautery is high, like enough volts to cause a
spark (10,000). The electrons go through the heart. In fact you can bovie on the heart and
not cause VT or VF. How could that be?
The frequency of the heart is 1 to 2 hertz. 1 Hertz is 1 cycle per second which would be a
heart rate of once per second or 60 times a minute. 2 Hertz would be a heart rate of 120
beats a minute. The frequency of the electrocautery is 1,000,000 hertz (1 MHZ) (one
E0-&"0('OT5$/"0$@#>,0$+#0%*='$")('$3#)($"0&('$@01&)%0$,'$)%0% MEGAHURTS. It is too
high a frequency for the heart to track and have VT or VF. If you turned the frequency of
the bovie down to 1 kilohertz (1,000 hertz) it would cause VT, VF, and death
immediately.
Bird (not to scale)
Electric Bus
Rubber Tires on
Road
Electric Train
Steel Wheels
Steel Track
Why does an electric bus have two wires and an electric train just have one wire?
The electric bus has rubber tires. The electro*%$1&*='$-0'$#88$'"0$@)%$@0cause the tires
insulate. Electrons need a way back to the power station. The bus must have TWO wires
to work.
The electric train has one wire because it uses the track as the ground. The electrons go
from the power source to the wire to the train motor to the steel wheels to the track back
to the power source to make a circuit.
What would happen if I touched the electric train track? (NOT THE THIRD RAIL ON A
SUBWAY). Nothing. Because the track is grounded. You are grounded. There is no
voltage potential.
What would happen if a bird landed on the overhead wire? Would we get Kentucky Fried
Birdy? Nope. The bird would be fine. He has no voltage potential difference. He is at the
voltage of the wire, but there is nowhere for the electrons to go, so he is fine. If the bird
peed on the ground and made a circuit, then you get crispy bird. But just sitting on the
20
wire, he is fine. I want you to do some homework. The next time you are near an electric
train or bus look under the wire and see if there is a pile of crispy fried birds and
squirrels. I bet you wo*='$8,*+$&$%,*-20$#*05$Y#$9#'0*',&2$+,880(0*10?$*#$82#!$#8$0201'(#*%?$
no current, no crispy fried bird.
Now that you understand electronic circuits please study the following diagram.
Pacemaker
Atrium
Ventricles
You are in the OR at the end of a cardiac case. The patient is in third degree heart block
and the pacer is set to DDD. The resident removes the drapes and the ioban and one of
the ventricular pacing leads is pulled out. The heart rate is zero, the blood pressure is
zero, you begin CPR. Now what?
a. Continue CPR until I figure out what to do.
b. Put on external pacing pads and hope they work.
c. Start isoproterenol if I can find the drug and remember the dose and whether it
actually works.
d. Call cardiology, hope they are in the hospital, and have them place a temporary
wire.
e. P',14$&$9&1,*-$!,(0$,*'#$'"0$9&',0*'=%$%4,*$%#E0!"0(05$/"0$"0&('$!,22$9&105$/"0*$
open up the sternum and replace the pacing wire.
/"0$&*%!0($,%$05$<8$3#)$%',14$&$9&1,*-$!,(0$,*'#$'"0$9&',0*'=%$%4,*?$'"0$0201'(#*%$!,22$8,*+$
the wire and complete the circuit. Electrons may be dumb, but they travel at close to the
speed of light and they can find the wire. You may need to turn up the ventricular current.
Alternatively, you could borrow an atrial lead as the ground until you have a new wire.
Or you could jumper across the grounds to use the atrial ground for both atrium and
ventricle.
21
OK, last but not least, the code. You now get to learn the secret code.
Code
AOO
What is it?
Atrial pace, no sense, no
inhibitions
AAI
Atrial pace, atrial sense,
inhibited by atrium
Ventricular pace, no sense, no
inhibit
VOO
VVI
DOO
DVI
DDD
Ventricular pace, ventricular
sense, ventricular inhibit
Dual pace, no sense, no
inhibitions
Dual pace, ventricular sense,
ventricular inhibit
Dual pace, dual sense, dual
inhibit
Who gets it?
Sick sinus syndrome with intact
conduction in the operating room
with bovie. Ie Cardiac case, in OR,
with bovie, with heart rate low from
narcotics.
Sick sinus syndrome with intact
conduction system.
Third degree heart block in OR with
atrial fibrillation. Why atrial
8,@(,22&',#*.$B01&)%0$3#)$1&*='$
effectively pace the atrium if it is
fibrillating.
Third degree heart block with atrial
fibrillation.
Third degree heart block in OR with
bovie.
Third degree heart block with
supraventricular tachycardias
Third degree heart block.
What it does with sensing information
Area Sensed
Area Paced
How do I remember the code? The code is for a PACEMAKER. The first letter is the
area PACED. Why? Because it is a PACEMAKER. Suppose you saw a code ODO? Have
you ever seen a code ODO on a PACEMAKER? Well, what would an ODO be? It would
be a machine that just sensed and did nothing. An ODO is an ECG machine. The first
letter is the area paced. The second letter is the area sensed. The third letter is whether is
inhibited or not. O is do nothing.
DOO and VOO modes are ONLY FOR USE IN THE OR WITH BOVIE! They are the
Paris Hilton of pacing modes: no sense, and no inhibitions.
I hope you learned something. If you have questions or are confused or find an error
please send me an email.
22
Arthur Wallace, M.D., Ph.D.
VAMC Anesthesiology (129)
4150 Clement St.
San Francisco, CA 94121
Phone: 415-750-2069
Pager: 415-210-6077
Fax:
415-750-6946
E-mail: awallace@cardiacengineering.com